Optical recording media such as CDs (Compact Discs) and DVDs (Digital Versatile Discs) have been widely used as information recording media. Further, recent years have seen the ongoing diffusion of a type of optical recording medium referred to as Blu-ray Disc (Registered Trade Mark; hereinafter called “BD”). The BD employs, for recording/reproducing operations, a blue or violet laser beam that is shorter in wavelength than a conventional one as well as an objective lens greater in numerical aperture NA than a conventional one. The BD is thus capable of storing a larger amount of information than before. More specifically, such optical recording media being widely used are compatible with a reproducing apparatus or recording/reproducing apparatus that employs, for recording/reproducing operations, a laser beam of a wavelength of 405 nm (in the range of from 375 to 435 nm) and an objective lens of a numerical aperture NA of 0.85 (in the range from 0.7 to 0.95) for irradiation of the optical recording media with the laser beam.
Note that the so-called BD optical recording medium has tracks formed at a track pitch of 0.32 μm (in the range of from 0.1 to 0.5 μm). The so-called BD optical recording medium is largely divided into a BD-ROM on which data cannot be written or rewritten, a BD-R on which data can be written once, and a BD-RE on which data can be rewritten.
Generally, a tilt in the optical recording medium due to a warp or the like would cause the deformation of a beam spot of the laser beam at an information layer to broaden the beam spot, thereby inducing an error. The larger the numerical aperture of the objective lens, the wider becomes the laser beam width in the optical recording medium. Thus, the deformation of the beam spot caused by the tilt is likely increased. As described above, the deformation of the beam spot resulting from the tilt is likely increased because the so-called BD optical recording medium employs an objective lens with a numerical aperture NA greater than a conventional one. On the other hand, suppose that the information layer is located closer to the light incident surface (the surface of the optical recording medium on the cover layer side). In this case, since the optical path length of the laser beam from the light incident surface to the information layer is shorter, the beam spot of the laser beam is less deformed on the information layer even at the same tilt of the optical recording medium. Accordingly, to reduce errors caused by the tilt of the optical recording medium, the information layer is located preferably as close to the light incident surface as possible. The so-called BD optical recording medium requires the information layer to be located within the range of 120 μm or less from the light incident surface, and is thought to be provided with the information layer preferably within the range of 110 μm or less from the light incident surface.
Meanwhile, foreign matters such as fingerprints (sebum) or dust particles adhered to the light incident surface would cause the foreign matter to be reflected on the beam spot of the laser beam on the information layer, thus sometimes inducing an error. Even with foreign matters of the same size, the more closely the information layer is located to the light incident surface, the greater the percentage of the portion of the beam spot on which the foreign matter has effects becomes. Accordingly, to reduce errors due to the foreign matter on the light incident surface, the information layer is located preferably as far away from the light incident surface as possible. The so-called BD optical recording medium requires the information layer to be located within the range of 40 μm or more from the light incident surface, and is thought to be provided with the information layer preferably within the range of 50 μm or more from the light incident surface.
For the reasons mentioned above, the so-called BD optical recording medium is provided with a cover layer thinner than the substrate and an information layer located within the range of a predetermined distance from the light incident surface. This allows for reducing those errors that would be otherwise caused by the tilt of the optical recording medium and by the foreign matter on the light incident surface. To be more specific, the optical recording medium, which has the information layer located at approximately 100 μm from the light incident surface, is being spread.
Furthermore, the optical recording medium can include a plurality of information layers between the substrate and the cover layer with a light-transmitting spacer layer sandwiched between the adjacent information layers, thereby increasing the recording capacity. Note that to record data on the optical recording medium having a plurality of information layers, the recording laser beam is focused on a target information layer to be recorded on, thereby data can be selectively recorded on the target information layer. Furthermore, the reproduction laser beam is focused on a target information layer to be reproduced from, thereby data can be selectively reproduced from the target information layer. A suggestion has been also made to the structure of the so-called BD optical recording medium in which a plurality of information layers are included. A dual layer optical recording medium of BD is being spread which has an L0 information layer (a first information layer) located at approximately 100 μm from the light incident surface and an L1 information layer (a second information layer) located at approximately 75 μm from the light incident surface.
Another suggestion has been also made to the structure of the so-called BD optical recording medium in which three or more multi-layers are included. Even in the case of the three or more multi-layer structure, the all three or more information layers are preferably located within the range between 40 μm and 120 μm from the light incident surface.
However, provision of three or more information layers within such a microscopic region sometimes raises a problem of crosstalk during reproduction. More specifically, the reproduction laser beam for irradiating the optical recording medium is reflected not only on the target information layer from which data is reproduced, but also part of the reproduction laser beam is reflected on another information layer in various manners. Thus, the part of the reproduction laser beam may have effects on the signal beam reflected on the target information layer to be reproduced from. The crosstalk is thought to be divided mainly into two types; inter-layer crosstalk and confocal crosstalk (for example, see Japanese Patent Application Laid-Open No. 2004-213720 and Japanese Patent Application Laid-Open No. 2006-73053).
The inter-layer crosstalk refers to a phenomenon in which crosstalk light reflected on an information layer other than the target information layer to be reproduced from has effects on a signal beam reflected on the target information layer to be reproduced from. This phenomenon may occur not only with the three or more multi-layer optical recording medium but also with the dual layer optical recording medium. For the three or more multi-layer optical recording medium, the inter-layer crosstalk is thought to be reduced by the plurality of spacer layers being formed in mutually different thicknesses. Furthermore, the greater the thickness of the spacer layers between the information layers through which the crosstalk light passes, the more the inter-layer crosstalk is reduced. It is generally considered that the spacer layers with a thickness of 15 μm or greater can reduce the inter-layer crosstalk substantially to an insignificant level.
On the other hand, the confocal crosstalk refers to a phenomenon in which the optical path of a signal beam reflected only once on the target information layer to be reproduced from will coincide with the optical path of the crosstalk light reflected in another manner. This phenomenon is sensed at a photodetector as if both the beams were reflected at the common focus though the focal positions of the optical paths are actually different from each other in the optical recording medium. This phenomenon occurs with the three or more multi-layer optical recording medium. The thicknesses of each layer of the optical recording medium vary microscopically in its circumferential direction, thus portions highly affected by the confocal crosstalk and portions less affected by the confocal crosstalk coexist in the circumferential direction. This may raise fluctuation in reflectivity per one revolution, thereby causing degradation of accuracy in reproducing data. This confocal crosstalk is also thought to be reduced by the plurality of spacer layers being formed to have thicknesses different from each other.
However, to prevent both the inter-layer crosstalk and the confocal crosstalk, each spacer layer has to be thick enough or more than enough to prevent the inter-layer crosstalk. Simultaneously, to reduce the confocal crosstalk, each spacer layer needs to be different from each other in thickness. In other words, some spacer layers are required to have a thickness that is more than enough to prevent the inter-layer crosstalk. Such an increase in thickness of the spacer layers would cause the stack density of information layers to be reduced in the direction of thickness, thereby decreasing the number of information layers available.
Note that the number of information layers available can be presumably increased by extending the range of the information layers away from the light incident surface. However, in this case, the aforementioned errors due to the tilt of the optical recording medium would more likely occur. It can also be thought to increase the number of information layers available by extending the range where the information layers are located towards the light incident surface side. However, as described above, the errors due to the foreign matter on the light incident surface would more likely occur.